Problem: $ { {-2} \times \left[\begin{array}{rr}{2} & {1} \\ {2} & {1} \\ {1} & {-1}\end{array}\right] = {?} } $
To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{-2}\times{2} & {-2}\times{1} \\ {-2}\times{2} & {-2}\times{1} \\ {-2}\times{1} & {-2}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{-4} & {-2} \\ {-4} & {-2} \\ {-2} & {2}\end{array}\right]}$